With the evolution of technology in the Electric Power Generating industry, battery systems came to play a critical roll in providing back-up energy in emergency situations. When emergencies occur, it's essential that the battery systems perform as designed or serious consequences result because of failure or partial failure of the battery system.
In the past, maintenance programs were designed to evaluate batteries and battery systems under static conditions. Although conscientiously implemented, these methods proved ineffective. Specific Gravity Readings, Cell Voltage Measurements and Electrolyte Level Maintenance provide information regarding potential, but do not accurately evaluate operational capability. Using only these methods, batteries, and battery systems can appear perfectly sound, but fail when called upon to supply emergency power.
In 1968, the Institute of Electrical Engineers (IEEE) became active in the development of standards for batteries and quickly discovered accepted practices to be ineffective. In depth studies determined that the only positive means to evaluate the operational integrity of battery systems is discharge testing. Exercising systems under actual operating conditions is the only method of assuring all components functionally capably.
Recognizing a need, the IEEE prepared documents to be used as industry standards to improve methods of battery evaluation.
In addition to previous practices, discharge testing, at specific time intervals, was presented as a necessary test procedure to enhance the maintenance/test programs, since it's desirable to monitor battery systems after they are connected as emergency power sources. Monitoring should be on a continual basis and should detect weaknesses in the operational integrity of the system, so preventative maintenance can be instituted. Past methods of monitoring although appearing sound have also been proven ineffective, resulting in personnel injury and exorbitant costs to the power industry. Experience has shown that equipment presently used monitors only static conditions and does not determine operational integrity. The parameters monitored consist of Overvoltage, Ground Faults and Undervoltage conditions and although valid indications of potential faults, one critically important ingredient is missing, that of Load Testing. Most battery, battery strap and connection problems can only be detected while current is flowing. It is therefore essential that periodic load tests be performed.
The first major power company to recognize load testing as an essential feature for battery system monitoring was Louisiana Power & Light. In 1980, a load test was implemented by Louisiana Power & Light personnel as the initial step to evaluate system integrity. That is, such load tests looked at the overall voltage of the entire battery system to show that the entire battery system could supply the proper output. This was not a reliable test. Although sound in theory, their approach to this problem was not totally effective. The concept certainly addressed the need for exercising battery systems under actual operating conditions and detection of faults resulting in an open loop situation. The test method did not however, detect system degradation,therefore potential failures went unnoticed. This lack of detection resulted in a major failure which threatened the life of a maintenance man and caused a substation melt down.
Also, in the past, batteries were three-cell units, 100 Ah, lead-calcium battery, a standard for many year making up about 80% of the station batteries in service today. This battery utilizes a leaded on/flag-type terminal for interconnection with 1/4-inch hardware. It is, however, subject to deterioration that is not normally visible. This deterioration results from the growth that takes place with the positive plates as the batteries age. In this particular battery, the seal formed by the o-ring between the terminal and case is broken allowing fumes and electrolyte to migrate up into the leaded joint that provides the post-to-terminal connection. This particular type of deterioration starts when the terminal to the case seal is broken by forces produced by positive plate growth. This growth is a continual process throughout the life of a lead-acid battery. In later models, battery manufacturers have improved the case-to-terminal seals and cell designs to minimize these effects.
Newer criteria for battery specification include double connectors throughout, independently bolted hardware for all connections of 5/16-inch stainless steel; heavy-duty 1-inch square post, which normally requires the purchase of a battery with a minimum 150 Ah rating, which, in many cases, exceeds the sizing needed for the application. The battery post and terminals are now one solid 1" nominal piece, without any leaded connection points. The interrow or step connections are also double. The double strapping has independent bolting.
Battery chargers, when disconnected from the station batteries or bus, as in an open-strap battery, produced a variety of undesired results such as "0" volts, extreme over-voltage 150 V+, and normal voltage with 130 V AC ripple.
Also, in the past, the battery alarm system used to sense under-voltage, open strap, and battery grounds, was set up so that the battery ground and open strap conditions produced one common alarm output.
The failure mechanisms of cells and the 120 interconnections making up a 120 V station battery are difficult to detect. Deterioration of the connection points takes place over a long period of time, slowly reducing through oxidation and corrosion the contact surface areas until the connection area can no longer support the load.
Charge current necessary to float charge a 100 Ah lead-calcium cell at normal float levels is only 100 MA, or about 1 A for lead-antimony-type cells of the same size. Alarm output would not occur with the ripple level, positive charge, or current center tap balanced voltage, until after the connection became incapable of supporting this small charging current. If a dc system is called upon during the period of a developing connection problem, such test systems would not be satisfactory.
If failures occured, as with a sudden snip of a wire, all of the detection methods would in time detect the failure. It has been found that the majority of battery and battery-strap problems can only be detected while current is flowing out of the battery. It is therefore essential that periodic load tests under full loads be performed frequently enough to insure the integrity of the system being monitored.
In order to overcome the deficiencies in the prior art as set forth hereinabove, Applicant developed a monitoring device that could accurately detect potential failures as well as actual failures which is disclosed in parent application Ser. No. 475,321 filed Mar. 14, 1983 now abandoned. The monitoring device disclosed therein is a Stationary Battery Monitor that not only monitors typical parameters, but provides an automatically controlled load test that could detect system deterioration as well as actual fault conditions. This monitor is a means to eliminate catastrophic failures in power stations resulting from the lack of battery system integrity and provides a load testing means that is implemented by applying a resistive load to the D.C. bus, placing the battery charger in current limiting and the load across the battery. During the loading period of a few seconds, cell group voltages are measured and compared to a predetermined alarm level. Should any cell group voltage fall below the detected alarm level, the test is immediately terminated and the signal to the alarm output latched, requiring a manual reset to clear the alarm condition. Initiation of the load test can be accomplished remotely by supervisory control, locally by a start test push-button or automatically by a preset timer. Safeguards are provided to prevent the test load from being permanently connected. Dual element 30 amp fuses provide primary protection. The fuses are time delay elements which operate in a failure mode after 25 seconds. The normal 10 second load test does not change the time-current characteristic of these fuses. Four 5 ohm 750 w resistors connected in parallel provide the 100 amp load. These resistors can operate at 5 times rated capacity for 10 seconds. Should the load contactor remain closed for any reason, the resistors become fuses after several minutes.
In brief, the battery monitor system disclosed in parent Ser. No. 475,321 (now abandoned) is connectable to at least one battery with cell groups of at least one cell for detecting and signalling faults by periodic load tests for detecting weak cells and high total strap contact resistance. The battery monitor system includes initiating means, load means, first lead means, second lead means, detecting means, comparator means and signal means. The initiating means is used to initiate the test. The load means is connected to the initiating means. The load means provides a load test for detecting weak cells and high total strap contact resistance. The load means imposes a load that would cause current to flow at a predetermined level of a magnitude in order to load each battery to the in service battery demand requirements.
A first lead means is connected across each battery and each cell group within each battery. The first lead means is connected to the load means and the initiating means. The second load means is connectable to each battery and each cell group within each battery. The detecting means is connected to the second load means for detecting voltage outputs from each cell group. The comparator means is connected to the detecting means for comparing each output from each cell group with a present value for detecting weak cells or for detecting high total strap contact resistance in a cell group. The signal means is connected to said comparator means for signaling the presence of a weak cell or a high total strap contact resistance. The load means includes at least one resistor. Each resistor is sized to carry overloads for a predetermined in use time of a few seconds. The initiating means includes a contactor biased to an opened position with a drive coil timed to operate for a predetermined time equal to said predetermined in use time of the resistor.
At least one fuse is connected in the battery monitor system for fail safe use in a predetermined time greater than the predetermined in use time of the resistor. The comparator means detects weak cells and/or high total strap contact resistance in a cell group and the signal means signals weak cells and/or high total strap contact resistance in a cell group. The resistors are sized for fail safe use in a predetermined time greater than the predetermined failure time of this fuse and are of the edge-wound ceramic type. The time-delayed undervoltage contact prevents load test initiation when a low voltage condition exists. Should the fuses fail to open or the load test timer fail, the undervoltage contacts will drop the load after 60 seconds. Control circuitry and the load contactor are A.C. powered, insuring the load test feature inoperable should a loss of A.C. power occur. In addition, the monitor is connected to the power station's D.C. distribution panel through a circuit breaker providing another zone of protection.
The invention described in Ser. No. 475,321 appears to have been the only instrument in existance at the time of the filing thereof providing the means to automatically monitor battery system integrity dynamically and detect potential as well as actual fault condition.
The battery testing system described therein was designed to detect, perform integrity load tests, indicate and alert the central station via the supervisory control system, when battery system faults occur at the substation level. The entire system disclosed therein is self-contained for easy installation in the substation and continuously monitors abnormally high or low voltage conditions, an "earth ground" of less than 1000 ohms on either side of the battery system, and a loss of A.C. power to the charger. Battery and battery strap resistances are measured thereby during a 10-second 100 ampere load test performed automatically at pre-determined intervals. In addition, these tests can be initiated manually at either the central station or the monitor. Either 24 or 60 cell configurations can be monitored by the system.
The system described therein connects sense leads across the overall battery as well as cell groups within the battery. The system monitor senses the overall battery voltage continuously and alarms on either an abnormally high or sustained low voltage, which would signal a possible charger problem. The low overall voltage alarm takes normal load operation into account and requires that a low voltage condition be sustained for greater than one minute. An earth ground detector alarms if either the positive or negative side of the battery is connected to ground through less than 1000 ohms of resistance.
The system described therein further incorporates a unique integrity load test feature which is implemented by periodically applying a resistive load which draws approximately 100 amps for 10 seconds. While the load is applied, the voltage across each of ten groups of cells is measured and compared against an alarm level. The alarm level, which is adjustable, is typically set for somewhere between 0.5 volts to 1 volt below the normal cell group voltage for this type of load. This alarm level will detect weak cells or high total strap contact resistances for a group of cells greater than 5 milliohms. Load current is verified during the test by monitoring the voltage across the load resistors and alarms on loss of load current. These load test alarms are latched for ease of trouble-shooting. Contact closures are available for customer use for all of the alarms and can be used to trigger either a local alarm or connected to the supervisory control system. Any load test alarm must be reset from within the monitor. Any alarm that is sensed will stop a load test and prevent the start of a new test. Light emitting diodes mounted within the system monitor are used to identify the type of alarm as well as which group of cells failed the load test. The load test can be initiated either by a remote contact closure, such as from the supervisory control; by a local push-button switch; or by a customer programmable timer which can be adjusted for intervals from one and one-half days to ninety-six days. The system described therein is easily installed and maintained. Calibration adjustments are available for each of the alarm levels. There are available a test/calibration unit which can be plugged into the monitor in place of the battery test leads. These calibration checks are recommended once a year. To protect the customer's battery against any possible system load test failures, the following precautions have been provided in the system disclosed in Ser. No. 475,321. The load is switched by an A.C. fail-safe, heavy duty contactor. The load is timed off by a 10-second timer which is backed up by thermally operated fuses in each side of the load line. These fuses are designed to blow in approximately 25 seconds. The low voltage alarm will trip on a sustained load condition.